Specify behavior of popovers with fullscreen API

[nt1m]

Currently showModal() cannot be called on dialogs that are popovers to avoid conflicting elements in the top layer.

In a similar fashion, you shouldn't be able to request fullscreen on popovers. I assume this was an oversight since the fullscreen spec is separate.

[annevk]

cc @josepharhar @mfreed7

[josepharhar]

This PR prevents popovers which are showing from getting fullscreen: #204

[nt1m]

Fixed by #204